Jason Cornwell, The Official Gmail Blog:
We’re kicking things off with two new themes for you to try out as a sort of sneak peek at what we’re up to. Starting today, you’ll see the “Preview” and “Preview (Dense)” themes in the Themes tab in Gmail Settings. Why two themes? Our new interface will eventually expand dynamically to accommodate different screen sizes and user preferences, but until then you can pick the information density that you prefer.
Feels cleaner and more streamlined.